The invention relates to a method for selective electroplating of metals on metal or metallised products in an elongated strip by means of an apparatus in which a masking belt and the product strip together are guided against the periphery of a wheel or drum and along means for exposing the products to be plated to an electrolyte, whereby masking belt and product strip are both provided lengthwise with holes in a regular pattern, whilst drive means are provided for lengthwise transport of masking belt and product strip.
A similar method is proposed in U.S. Pat. No. 4,493,757. In this known method use is made of masking means, made of sections, which opposite each other are somewhat movable in the lengthwise direction of the masking means and whereby use is made of indexing pins which engage in transport holes provided in the product strip. By engaging the indexing pins in the transport holes it is aimed to obtain a correction for tolerance differences in the masking means and product strip due to inaccuracies during manufacturing and/or temperature difference between mask and product strip. A precise position of masking means in relation to the product strip is essential for accurately applying the metal deposits--mostly precious metal--on the predetermined positions. On the one hand such an accurate positioning of metal deposits, particularly of precious metals is desirable for economic reasons, on the other hand it can be required for further processing of the products, such as assembling the products in a printed circuit board by means of soldering or such whereby specific parts of the product should not be plated with precious metal but require a non-precious metal such as tin for solderability.
Technologies used hitherto and as described in the above mentioned patent were very satisfactory for the electroplating of leadframes for the production of integrated circuits and the like, which were stamped or photochemically etched and whereby the product strip consisted of an elongated strip of interconnected products.
Leadframes are for instance used to assemble a chip on a printed circuit board whereby such a leadframe contains a number of leads which on one end are connected with the chip and require a precious metal plating, mostly gold or silver, whereas the other end of the leads serves to connect the chip with e.g. a printed circuit board and usually requires a tin lead coating.
The increase of the number of functions of modern chip technology demands a dramatic increase in the number of connections to the chip and can go up to as much as 1,000. In addition there is a trend towards lighter weight of components.
To meet these requirements product strips have been developed whereby the metal products are supported by plastic film. In general such product strips with plastic film support are manufactured as follows:
a) Stamp transport or sprocket holes and windows in locations, where in the finished product strip no support of the metal products by the plastic film is desired, into a plastic film, for instance of poly-imide or polyester with a thickness of 0.05-0.15 mm to which a dry adhesive film of approximately 0.025 mm had been laminated. PA1 b) Laminate a metal foil, usually a copper foil with a thickness of 0.035 mm to the prestamped plastic adhesive film. PA1 c) Apply a photo-sensitive liquid or dry resist to the metal foil, expose and develop the resist to the desired pattern of the metal product. PA1 d) Chemically etch the metal foil so as to obtain the desired metal product pattern and strip the remaining photoresist to obtain a bare metal product surface.
An alternative method for manufacturing a product strip composed of metal products supported by a plastic film is obtained by direct laminating the metal foil, mostly a copper foil, with a plastic film without an intermediate adhesive layer. In this case the transport holes and windows in locations where the metal products must not be supported by the plastic film are obtained by photochemical etching of the supporting plastic film. The metal products are then formed as described above to obtain the product strip.
The described production processes can provide extremely delicate leadframes with a very high lead count. Such product strips are known in the industry as TAB films (TAB=Tape Automated Bonding).
A product strip obtained this way and consisting of a very thin plastic film, supporting the leadframe products formed there-on is very vulnerable and it will be clear that exposing the strip to high pulling forces will lead to unacceptable deformation of the leadframes. For this reason it was customary up to now to electroplate, usually with gold, the products all-over, which resulted in high cost, whilst as already indicated above, it not only is cheaper but also quality wise much better when parts of the leads are tin lead coated for making solder connections to a printed circuit or the like, instead of goldplated which after soldering often shows brittle solderjoints.
The industrial demand for these delicate leadframes with supporting plastic film is strongly increasing as is confirmed in "Electronic Packaging and Production", August 1986: "TAB rebounds as I/O increase" and in "Electronlc Materials Report" February 1990: "Smart cards to fuel TAB growth".
In connection with the increased demand for TAB film structures it becomes important for the electroplating of these films to provide a system in which a masking belt and a product strip can be positioned accurately towards each other, without exposing the product strip to undesirable forces, so that precise electroplating of selective areas of the products can be effected, also when extremely delicate products, such as TAB film structures must be processed.